The present invention relates generally to turbine meters, and more particularly to improvements in a turbine meter in which a pressure-fed gas bearing is used in the shaft bearing of its vane wheel or rotor, and which is highly suitable for measurements of flow velocities or flow rates particularly of gases and the like having no lubricative property.
In general, turbine meters are used for measuring flow velocities, flow rates, and other quantities of fluids flowing in pipes and ducts of large inner diameter such as smoke stacks and flues.
In a conventional turbine meter of this type, a ball bearing or a sleeve bearing is used for the bearing supporting the shaft of the vane wheel or rotor which is rotated by the flow of a fluid.
However, while ball bearings have the advantage of relatively low rotational resistance, they have the drawback of poor durability. Particularly in cases such as that where a mechanism for supplying lubricating oil to the bearing cannot be installed because of miniaturization of the turbine meter, itself, or that where the fluid to be measured is a high-temperature or low-temperature gas and there is no suitable lubricating oil which can be used, the ball bearing rotates in an unlubricated state. For this reason, a ball bearing under such conditions have an extremely short serviceable lives.
Sleeve bearings have the advantage of better durability than ball bearings but have the disadvantage of high rotational resistance. For this reason, the measurement accuracy of a turbine meter using a sleeve bearing becomes poor in instances of measurement of gases of low density which cannot exert ample torque to the vane wheel. Furthermore, in the measurement of a gas, damage such as seizure, galling, and scoring easily occur in the sleeve bearing since the gas does not have lubricative property.
Still another problem accompanying both ball bearings and sleeve bearings, when used in a turbine meter, is that when foreign matter such as dirt, dust, and slurry are in a mixed state within the fluid being measured, these foreign substances readily infiltrate into the bearing and give rise to a variation in the rotational resistance, whereby the measurement accuracy drops, and the instrument error characteristic deteriorates. Furthermore, the friction in the bearing increases, and the serviceable life of the bearing thereby becomes short, whereby stable measurement over a long period becomes difficult.
Accordingly, for the purpose of solving the above described problems, turbine meters in which so-called pressure-fed air bearings are used for rotatably supporting the vane wheel have been developed. These known turbine meters are of so-called in-line type of a construction wherein an outer cowling or casing of the same inner diameter as the pipe line or conduit conducting the fluid to be measured is provided, and a large-diameter vane wheel for rotating within this outer casing is rotatably supported on a pressure-fed are bearing. In the case where the pipe line conducting the fluid to be measured is of large diameter, however, the structure of the pressure-fed air bearing of this in-line type turbine meter is large, whereby the quantity of air discharged therefrom and becoming mixed with the fluid being measured is not negligible and greatly impairs the accuracy of measurement.
Furthermore, in a known turbine meter of this type, it is necessary to provide through holes at two places in the wall of the outer casing respectively for piping to supply air for the pressure-fed air bearings and for a pickup for detecting the rotation of the vane wheel. As a consequence, the construction of the meter is complicated, and the work of assembling this meter is laborious.
Still another problem accompanying a known turbine meter of this type is that, in the case of measurement of a fluid containing much foreign matter such as dirt, dust, and slurry, this foreign matter adheres to parts of the meter such as the vanes of the vane wheel and the bearing and thereby gives rise to a drop in measurement accuracy.